Gas chromatography is performed in a special instrument where a small amount of liquid mixture is injected into an apparatus where it is volatized in a heated chamber. The volatized mixture is swept through a column in a stream of gas, such as helium or neon under conditions where its components separate into pure compounds. The column is located in a heated oven in order to facilitate the separation. Just before each compound exits the instrument, it passes through a detector, which sends an electronic message to the recorder, which responds by printing a peak on a piece of paper identifying the compound.
Typically the column is heated by placing the column in an oven. The heat facilitates compound separation by raising the column temperature and speeding up the compounds in the mixture. For precise work, column temperature may be controlled to within tenths of a degree. The optimum column temperature is dependant upon the boiling point of the sample. Generally, a temperature slightly above the average boiling point of the sample results in an elution time of 2-30 minutes. If a sample has a wide boiling range, then temperature programming can be useful. The column temperature is increased (either continuously or in steps) as separation proceeds.
Accordingly, analytes of interest are assayed at different temperatures, including high temperatures such as 500° C., and it is necessary to cool the oven and the column prior to testing additional samples. Long cooldown periods are problematic because they lengthen the sample cycle time reducing instrument productivity. Delay is compounded in high throughput analysis where a gas chromatograph is needed to analyze a large number of samples containing the same or different analytes of interest. Users waste time waiting for the column and oven to cool prior to running additional samples.
Ventilation systems including fans have been added to the gas chromatograph to blow air into the oven and onto the column between runs. However, conventional designs are slow to cool for there are considerable flow restrictions which impede the air flow throughout the oven. Furthermore, certain designs allow the cooling inlet air to mix with exhausting air resulting in a slower, less efficient cool down of the oven. Moreover, temperature gradients may form in the oven reducing the consistency or uniformity of the cooling down components.
Prior art of interest includes one system which relates to a chromatography oven which includes a fan within a housing adjacent to rear walls, an ambient air intake vent in the rear wall, and an exhaust vent within a rear corner of one of the side walls adjacent to the rear wall for exhausting the tangential flow of air created by the rotating fan. However, this design has considerable flow restrictions which impede the air flow throughout the oven resulting in a less efficient cool down.
Of further interest is another prior art system which relates to an apparatus having a first compartment including a chromatography oven with fan for circulating heated air over the columns while the oven is closed and for drawing in ambient temperature cooling air in the first compartment into the oven while the oven is open. Ambient air is drawn into a tortuous path in the first compartment. Cooling air from the second compartment flows into the first compartment via openings in the baffle. The cooling air flows over the oven exterior and is at least partially drawn into the oven by an oven fan while the oven is open. The oven heater, coaxial with the blades, is located between an oven wall and blades. A ring baffle, having approximately the same diameter as and coaxial with the blades, is located between the wall and the blades. A fan outside of the oven draws air from the oven through an outlet while the oven is open. The second fan is separated from an inlet for the oven by a baffle having an opening through which air is drawn by the second fan while the oven is closed. The second compartment includes a casing for fluid flow controllers for the columns, which casing is maintained at constant temperature by ambient air drawn around the second compartment. However this design impedes air flow because the air entering the oven counters air flow leaving the oven reducing the efficiency of the cooling. Furthermore, the baffle impedes airflow and produces a temperature gradient in the oven which results in a less efficient cool down. Moreover, this device requires two fans to circulate airflow which takes up additional energy and is noisy.
Of further interest is another prior art system relating to gas chromatography (GC) system employing a low-thermal-mass oven in which intake and exhaust vent apertures are aligned with respect to the rotational axis of the stirring fan. The poppets of the vent dynamically vent to ambient the air-flow generated by the stirring fan. The geometry of the vents cooperates with the axial and radial components of the stirring fan to promote conical vortex air flow, to facilitate mass-flow interchange with ambient air. However, the ventilation system includes a bulky vent servo in order to drive a carriage assembly which opens a front exhaust poppet. Exhaust leaves the front of the oven never circulating back over the oven skin resulting in reduced efficiency.